/Engine/Source/Runtime/InteractiveToolsFramework/Public/BaseGizmos/GizmoElementBase.h
For an element hierarchy representing multiple parts of a single gizmo, the part identifier establishes a correspondence between a gizmo part and the elements that represent that part. The recognized part identifier values should be defined in the gizmo. Gizmo part identifiers must be greater than or equal to one. Identifier 0 is reserved for the default ID which should be assigned to elements that do not correspond to any gizmo part, such as non-hittable decorative elements.
virtual void SetPartIdentifier ( uint32 InPartId )